Single unit controlled release tablet dosage forms of drugs have known disadvantages compared to the more desirable multiple unit dosage forms. In single dosage matrix tablets the active ingredient is dispersed as a solid in an inert, release delaying matrix. The embedding matrixing material that slows the release of the drug is usually an insoluble plastic (e.g. methyl acrylate-methacrylate, polyvinyl chloride, or polyethylene), a hydrophilic polymer (e.g. methylcellulose, hydroxypropylmethylcellulose, or sodium carboxyl methylcellulose), or fatty compounds (e.g. various waxes, such as carnauba wax, or glyceryl stearate). Such tablets pass undisintegrated through the gastrointestinal tract, and become ever smaller due to breakdown (erosion tablets), or wherein the active compound is released only in the intestine (enteric-coated tablets), in either case depleting the surface inward through the matrix by dissolution or diffusion through the matrix to the external solution.
Multiple unit dosage forms are usually contained within a rapid dissolving capsule, or are compressed into a tablet, and soon after ingestion upon its dissolution fall apart into the multiple units, such as pellets. The sub-units or pellets have the desired slow release properties of the dosage form. In contrast to the matrixed formulations, the multiple unit pellets are each surrounded by a polymeric delay release film coating that provides the controlled, slowed release of the active ingredient from within the pellet. These coated dosage forms are generally referred to as "reservoir devices" when diffusion phenomena occur through the release slowing coat, or "encapsulated formulations" when the slow release coat slowly dissolves (see e.g. Remington's Pharmaceutical Sciences, Mack Publ. Co.). In the case of the single unit, matrixed delayed-release dosage forms, considerable variations can occur in the gastric residence times, and this can lead to an irregular passage through the gastrointestinal tract and thus to differing, fluctuating blood level values. Local irritation also becomes of lesser concern also being reduced by the use of multiple unit pharmaceutical dosage forms. "Dose dumping" or the too rapid release of the active compound from delayed-release dosage forms (see e.g. H. Blume, "Biopharmazeutische Aspekte von Multiple Unit Dosage Forms; ein Vergleich nit Single Units" Biopharmaceutical aspects of multiple unit dosage forms; a comparison with single units!, published by Capsugel, Basel, and delivered at a symposium in November 1988 in Hamburg).
A variety of pharmaceutical formulations having delayed-release coatings are described, for example, in EP No. 147,780, in the form of tablets and granules which contain an active compound core and are coated with a film of polyvinyl alcohol.
In EP No. 634,366 an oral dosage form for delayed release tramadol preparations is disclosed wherein the active compound is either embedded in a delayed-release matrix of hydrophilic or hydrophobic polymers, long-chain fatty acids or fatty alcohols and one or more polyalkylene glycol. Film-coated spheroids are also described in which the active ingredient is embedded in a "spheronizing" material, such as in microcrystalline cellulose, and coated with a release controlling film.
In EP No. 731,694 (WO95/14460) delayed-release opioid formulations, for example pellets, are described which provide an analgesic effect over at least 24 hours after oral administration. The pellets have of an active ingredient-containing core which is covered with a polymer film suitable for delaying the release of the active. In addition to the polymers the film contains an acid-soluble compound and a plasticizer. The use of plasticizers, however, can be most disadvantageous, because these migrate out of the membrane, and this composition change can affect the release of active compound during storage. EP No. 147 780 describes on page 2 that a chemical interaction between plasticizer and the active compound can also take place and this leads to a reduction in the shelf life of the product.
It is mentioned in Sucker, Fuchs, Speiser "Pharmazeutische Technologie" Thieme Verlag Publisher. Stuttgart, 1978, that beside its actual film-improving property, the plasticizer affects water vapor permeability and the disintegration. The admittedly low but noticeable vapor pressure furthermore leads to an evaporation of the plasticizer, resulting in an alteration of the physical properties, such as the dissolution rate of the film.
Pellet formulations are distinguished by their large surface area because of the presence of many, often a few hundred sub-units per therapeutic dose. In the case of readily soluble substances such as tramadol hydrochloride, this can lead to a need for delayed release material, which undesirably reduces the active ingredient content of the pellet formulation. This is particularly undesirable in the case of actives which are administered at relatively high doses, because the quantity of pellets necessitates the use of a larger hard gelatine capsule. This is therapeutically undesirable, because it can be unpleasant for the patient.